1. Field of the Invention
This invention relates to an apparatus and method for measuring the surface and subsurface level of moisture within and permeability and/or porosity of enamel, dentine or tooth tissues by measuring the resistance or conductivity of the enamel, dentine or tooth tissue and its rate of change with time. The method and apparatus according to the invention can also be used to determine tooth sensitivity.
2. Discussion of Prior Art
The moisture levels of tooth tissue vary depending on external and internal parameters. Externally the moisture level depends on the level of fluid in the environment (humidity) and the type of tooth tissue being tested, enamel being a dry highly crystalline material and dentine being a collagenous material with a higher moisture level.
A measurement of moisture level in enamel, dentine or tooth tissues is of value in determining the efficacy of adhesive bonding agents for tooth filling materials.
Dentine also has a large number of tubules passing from the inner pulp to the junction between enamel and dentine. Fluid can pass along these tubules and tooth pain and sensitivity are attributed to this flow which may be induced by application of heat, cold and solutions which change the osmotic potential. A number of dental treatments related to the reduction of tooth sensitivity aim to block the dentinal tubules and reduce the fluid flow. A measurement of the level and rate of change of surface moisture level relates to the flow of fluid through dentine tissue and tubules and is therefore of use in providing a means of predicting the sensitivity of teeth or the efficacy of treatment designed to reduce sensitivity.
The moisture level can also be used to give an indication of how close the pulp tissue is to the dentine being measured. This information is valuable in determining how to treat the tooth.
Devices are known that measure the electrical resistance of teeth in the diagnosis of caries. Enamel is generally a good insulator, however, during the carious process, cavities form in the tissue which fill with saliva, the ions of which set up conductive pathways causing resistance values to fall. Thus, a measure of the conductivity of the tooth is an indication of the presence of caries. Such devices typically comprise an electrode which is coupled to a surface of the tooth that has been dried, and a second electrode which the patient holds in their hand. A summary of some of these known devices can be found in the paper xe2x80x9cRe-evaluation of Electrical Resistance Measurements for the Diagnosis of Occlusal Cariesxe2x80x9d by Ricketts et al. published in the British Dental Journal of Jan. 7, 1995.
Although such devices can be used for predicting the presence or absence of caries, the wide separation of the electrodes mean that it is difficult if not impossible to accurately predict the conductive path taken by the current. Thus, the device is not suitable for measuring the moisture level of specific areas of the tooth tissue. Furthermore, although it can be used for monitoring the large change in resistance associated with the presence or absence of tooth caries, it is not accurate or fast enough to detect smaller changes in conductivity.
U.S. Pat. No. 4,164,214 discloses an apparatus for measuring the sensitivity of teeth. This apparatus involves inserting a ground lead into the patient""s mouth and contacting the patient""s teeth with an oral probe. The electrical output of the oral probe is increased until the patient feels a tingling or warm sensation whereupon the patient acts to stop the increase, the value of the electrical output at this point is an indication of tooth sensitivity. This device relies upon the patient to record the moment they can feel the sensation. The level of feeling at which the patient stops the increase may vary from patient to patient and even from day to day with one patient. Thus, there is a disadvantageous level of subjectivity in the results obtained. The device relies upon nervous stimulation to operate which is a major disadvantage.
In accordance with one aspect of the present invention there is provided an apparatus for measuring the moisture level within enamel, dentine or tooth tissues, comprising: a probe having two electrodes arranged such that in use they both contact the enamel, dentine or tooth tissue under test; an electrical power source connected between the two electrodes; a sensor for sensing the voltage or current produced by said power source across or through the test sample; anda signal processor responsive to said measured voltage and/or current to calculate a value indicative of said moisture level within said enamel, dentine or tooth tissues, wherein the two electrodes are arranged coaxially with an insulator layer separating them and the diameter of the tip of said probe is less than 5 mm.
Thus, the present invention alleviates the disadvantages of the prior art by providing an apparatus comprising a probe having two electrodes that can be placed a known distance apart on the tooth tissue to accurately measure the conductivity or resistance and from this determine the moisture level of the tooth tissue located between the two electrodes. A measurement of the rate of change of conductivity with time provides an indication of the rate of change of moisture level and provides an objective indication of tooth sensitivity.
The two electrodes are arranged coaxially with an insulator layer separating them. This arrangement of the electrodes gives a compact easy to use probe which in use encloses a specific desired area of dental tissue between the two electrodes allowing the moisture level of this area of the tissue to be measured.
A small diameter of the probe at the contact surface, means that the moisture level in a small precise area can be measured. Furthermore, in such a probe the distance between the electrodes is small, thereby enabling areas of low moisture level and high resistance to be accurately measured.
In one embodiment the contact surface of the electrodes is flat, whereas in another the probe tip is convex. A good contact between the tooth tissue and electrode is important, thus it is advantageous for the contact surface of the electrodes to have a shape that is complementary to that of the tooth tissue to be measured.
In a further embodiment the tip of the probe is conical. Such a shape allows the insertion of the probe into the tooth tissue and thus this probe can be used for subsurface measurements.
In one embodiment the power source is an AC source, preferably a sinusoidal current source (although a square wave source could be used). An AC source reduces possible problems of distortion due to polarisation of the test sample. A current source means that only the resultant voltage need be measured.
In preferred embodiments the apparatus is bandwidth limited and the signals are filtered to reject extraneous interference. Thus, the apparatus may be made less sensitive to interference from other sources.
Advantageously the apparatus comprises a full wave rectifier, low pass filter and an analogue to digital converter to convert the analogue signal to a digital signal. The value of the digital signal provides a direct indication of moisture level.
In one embodiment the analogue to digital converter is a 16 bit or greater analogue to digital converter, preferably with a sampling rate of at least 100 Hz. A high resolution analogue to digital converter means that the apparatus is suitable for use across a wide range of resistances, typically from 100 Ohms to 100M Ohms and hence a wide range of moisture levels. A high sampling rate means that rapid changes in moisture levels can be accurately recorded.
Advantageously, the input impedance of the analyser is greater than 1 GOhms. A high input impedance of the analyser enables it to accurately measure moisture level even where the resistance of the tooth is quite high.
Preferably the apparatus comprises data storage means for storing calibration data. Thus, the apparatus can be calibrated for different probes and for different situations. It can for example be calibrated so that the resistance/conductivity measurements made by the apparatus are translated directly to moisture level. Alternatively the device can be calibrated so that the rate of change of these measurement can be translated to tooth sensitivity by measurement of rate of change of moisture level.
In one embodiment the signal processor of the apparatus is adapted to calculate tooth sensitivity from the variation in the measured values over time. The apparatus can thus be directly used to produce an objective measure of tooth sensitivity.
Preferably the apparatus comprises display means for displaying the calculated moisture level and/or tooth sensitivity, advantageously said display means is graphical and is adapted to display the moisture level as it varies with time.
In preferred embodiments the apparatus comprises an alarm that is set to be activated when the measured values exceed or fall below at least one preset value.
According to another aspect of the present invention there is provided, a method of measuring the moisture level of enamel, dentine or tooth tissues comprising the steps of: removing excess surface saliva from an area to be tested of the enamel, dentine or tooth tissues; contacting said test area with a probe having two electrodes; connecting an electrical power source between the electrodes; measuring the current or voltage produced by said electrical power source through or across said test area; calculating a value indicative of the moisture level of said test area from said measured values, wherein the two electrodes are arranged coaxially with an insulator layer separating them and the diameter of the tip of said probe is less than 5 mm.
In preferred embodiments the electrical power source is an AC power source and the method further comprises the step of converting the AC signals to DC signals. The use of an AC power source prevents the sample from becoming polarised, however a DC signal is easier to interpret, thus the method provides for the conversion of the signal.